System and method for controlling traction

ABSTRACT

A traction control system for a machine having an electric drive configuration is disclosed. The fraction control system includes an electric motor associated with at least one wheel and adapted to provide braking torque to the wheel. The control system further includes a controller configured to determine a rotational speed of the at least one wheel, compare the rotational speed to an allowable slide threshold, and adjust the braking torque to the at least one wheel during retarding if the speed is less than the allowable slide threshold.

TECHNICAL FIELD

This patent disclosure generally relates to a traction control systemand, more particular, to systems and methods for controlling tractionduring retarding for electric drive machines.

BACKGROUND

Vehicles having mechanical drive systems typically transmit torque totheir drive wheels via gear arrangements, which are commonly known asdifferentials. A differential typically transfers rotational motion froman input shaft to each of two wheels disposed on both ends of a driveaxle. Differentials are typically able to allow two wheels that areconnected to a single axle to rotate at different speeds. Conditionsrequiring such differential motion may occur when the vehicle is turningor when the two wheels are experiencing different traction conditions. Aloss of traction may result in a wheel sliding. Electric drive vehicleswith rear-wheel drive are susceptible to such loss of traction that maybe increased due to electric retarding applied to the rear wheels, whichrequires relatively higher ground friction than with all-wheel brakingsystems.

Even though differentials are effective in preventing wheel slipping orsliding for vehicles or machines, they are typically absent fromvehicles having systems driving each wheel independently from theothers, such as, vehicles having electrical or hydrostatic drivesystems. Such vehicles typically lack a direct mechanical linkagebetween drive wheels because each drive wheel is independently poweredby a motor that is associated with that wheel.

The disclosed systems and methods are directed to overcoming one or moreof the problems set forth above.

SUMMARY

The disclosure describes, in one aspect, a traction control system for amachine having an electric drive configuration. The traction controlsystem includes an electric motor associated with at least one wheel andadapted to provide braking torque to the wheel. The control systemfurther includes a controller configured to determine a rotational speedof the at least one wheel, compare the rotational speed to an allowableslide threshold, and adjust the braking torque to the at least one wheelduring retarding if the speed is less than the allowable slidethreshold.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 schematically illustrates an electric drive machine having acontrol system in accordance with an exemplary embodiment of the presentdisclosure.

FIG. 2 is a flow chart illustrating one embodiment of a method ofcontrolling traction in accordance with an exemplary embodiment of thepresent disclosure.

FIG. 3 is a flow chart illustrating one embodiment of a method ofcontrolling traction during retarding in accordance with an exemplaryembodiment of the present disclosure.

DETAILED DESCRIPTION

This disclosure relates to systems and methods for controlling tractionof an electric drive machine. An exemplary embodiment of a machine 100is shown schematically in FIG. 1. The machine 100 may be an off-highwaytruck, as shown, or any other vehicle that has an electric drive system,including passenger vehicles, trains, earthmoving machines, and miningvehicles. In an illustrated embodiment, the machine 100 includes anelectric drive system 102 operatively coupled to travel mechanisms 104to propel movement of the machine 100.

The travel mechanism 104 may include wheels and axles on each side ofthe machine 100. In the illustrated embodiment, the travel mechanisms104 include a set of front wheels 105 on each side of the machine 100and a set of rear dual wheels 106 on each side of the machine 100. Thetravel mechanisms 104 allow the machine 100 to travel on the surface ofa type of terrain, such as earth surface terrain. The travel mechanisms104 are shown as wheels, but it is contemplated that the travelmechanisms 104 may be any type of tractive or fraction mechanism known,such as, for example, tracks and belts.

The electric drive system 102 includes an engine 107, alternator 108,rectifier 110, inverters 112, 114, and motors 116, 118. The engine 107may provide power for the machine 100 and other machine components.Suitable engines may include gasoline powered and diesel poweredengines. In some embodiments, the engine 107 may be a diesel engine thatgenerates and transfers power to other components of the machine 100through a power transfer mechanism, for example, a shaft (not shown). Inthe illustrated embodiment, the engine 107 provides power to thealternator 108. The alternator 108 generates a three-phase alternatingcurrent, which produces electrical power.

In some embodiments, the rectifier of the electric drive system 102 mayconvert the three-phase alternating current to a direct current. One ormore of the inverters 112, 114 convert the direct current to alternatingcurrent to power one or more of the electric motors 116, 118. Theelectric motors 116, 118 represent motors that transfer the electricpower received from the alternator 108 into power that drives one ormore of the travel mechanisms 104. For example, in some embodiments, themotors 116, 118 may be wheel motors used to drive a wheel or wheels topropel the machine 100. In some embodiments, the rear dual wheels 106may be independently or directly driven such that each of the motors116, 118 may correspondingly drive each of the driven rear dual wheels106. A speed of the motors 116, 118 may be controlled by controlling thefrequency of the alternating current produced by the inverters 112, 114.

In some embodiments, a single motor drives all of the travel mechanisms104, while in some embodiments, a plurality of motors drives the travelmechanisms 104. In the illustrated embodiment, for example, an electricmotor 116, 118 is associated with each travel mechanism 104 embodied asthe rear dual wheels 106, including a right motor 116 and a left motor118. In some embodiments, the engine 107 may be used to power some ofthe plurality of motors, while a separate electric power source or powerstorage unit such as a battery (not shown) may be used to power theremaining of the plurality of motors. In some embodiments, the motors116, 118 may be driven directly from the separate electric power source.

The engine 107, alternator 108, rectifier 110, inverters 112, 114, andmotors 116, 118 may be operatively coupled to provide power sufficientto propel the machine 100 in a forward or a reverse driving directionduring a driving phase or propel phase of operation. When operating themachine 100 in the driving phase, the motors 116, 118 provide a propeltorque sufficient to propel the machine 100 in the forward or thereverse driving directions. In some embodiments, the electric drivesystem 102 may include a final drive (not shown), which includes aplanetary gear set connected between the motors 116, 118 and the travelmechanisms 104, to convert the speed of the motors 116, 118 into anappropriate magnitude of the propel torque to propel the machine 100 inthe forward or reverse driving directions.

Further, the electric drive system 102 may dissipate power sufficientlyto retard or provide braking to the machine 100 during a retarding phaseof operation. During the retarding phase of operation, the inverters112, 114, motors 116, 118, and a braking chopper 120, collectivelydefine an electric retarding system 122. When operating the machine 100in the retarding phase, the motors 116, 118 may provide a braking torquesufficient to cause the machine 100 to slow down and/or come to acomplete stop. In some embodiments, the motors 116, 118 during retardingmay generate alternating current that is converted to direct current bythe inverters 112, 114 and that flows through the brake chopper 120,which provides direct current to direct current conversion, and into abraking grid or resistor grid 124. In the illustrated embodiment, thepower that is generated by the motors 116, 118 during retarding may beused to power a fan 126 or other appropriate cooling system to reduce atemperature resulting from the heat energy radiating from the brakinggrid 124.

In some embodiments, the machine 100 may also include a braking system128 that includes the electric retarding system 122 and one or moreservice brakes 130, 132 for retarding or braking the movement of themachine 100. In some embodiments, the braking system 128 and the one ormore service brakes 130, 132 may be associated with corresponding travelmechanisms 104. In some embodiments, the braking system 128 and the oneor more service brakes 130, 132 may be associated with the front wheels105 and/or the rear wheels 106. In the illustrated embodiment, thebraking system 128 includes the electric retarding system 122 and theone or more service brakes 130, 132 embodied as a right service brake130 and a left service brake 132. The service brakes 130, 132 may behydraulic friction, hydro-mechanical, or mechanical brakes.

In some embodiments, all of the braking required to reduce a speed ofthe machine 100 may be provided by the electric retarding system 122. Insome embodiments, all of the braking required to reduce the speed of themachine 100 may be provided by the service brakes 130, 132. In theillustrated embodiment, if the electric retarding system 122 is notcapable of providing all of the braking required, a portion of thebraking required to reduce the speed of the machine 100 is provided bythe electric retarding system 122 and a portion of the braking requiredto reduce the speed of the machine 100 is provided by the service brakes130, 132.

The service brakes 130, 132 may be manually actuated by an operator,which also allows the operator to manually control the speed of themachine 100. In some embodiments, the service brakes 130, 132 may bemechanically, electro-mechanically, hydraulically, pneumatically, oractuated by other known methods. In the illustrated embodiment, theservice brakes 130, 132 may be automatically actuated by a controlsystem 134. In some embodiments, the control system 134 may determine anappropriate ratio of retarding torque splits between, for example, theleft and right set of dual wheels 106, or between the rear wheels 106and the front wheels 105. In other words, the portion of brakingprovided by the electric retarding system 122 may be split between theleft and right travel mechanisms 104 and/or between the rear dual wheels106 and the front wheels 105.

In the illustrated embodiment, the control system 134 may be incommunication with the electric drive system 102 through a data linkinterface 136. Additionally, or alternatively, the control system 134may be in communication with the electric drive system 102 and othermachine components wirelessly or remotely. In some embodiments, thecontrol system 134 may send a command to the one or more components inresponse to signals collected and transmitted from one or more sensors.The control system 134 may receive sensor signals directly from the oneor more sensors or indirectly such as, for example, from the data linkinterface 136. In the illustrated embodiment, the one or more sensorsinclude one or more speed sensors 138 that may measure, collect, andtransmit signals to the control system 134 indicative of the speed ofthe machine 100.

The speed sensors 138 may send speed signals to the control system 134in response to requests, or the speed sensors 138 may be configured tosend speed signals periodically, or in response to a machine event, suchas an increase in speed, or a deceleration, and other such events. Insome embodiments, the speed sensors 138 may measure a rotational speedof an axle used in the travel mechanisms 104 or other drive traincomponents that are associated with a ground speed (or linear tirespeed) of the machine 100. In some embodiments, the speed sensors 138may be capable of measuring an actual ground speed or travel speed ofthe machine 100. In some embodiments, the speed sensors 138 may beconfigured or arranged to measure a rotational speed of idling wheels.For example, in the illustrated embodiment, the idling wheels are thefront wheels 105. In some embodiments, the ground/travel speed may bedetermined by measuring the rotational speed of each idling wheel 105and calculating the average of the measured speeds.

In some embodiments, the speed sensors 138 may be configured or arrangedto measure a rotational speed of the powered or driven wheels. Forexample, in the illustrated embodiment, the powered or driven wheels arethe rear dual wheels 106. The rotational speed may also berepresentative of a rotating machine RPM. In some embodiments, the speedsensors 138 may be capable of sensing the direction of rotatingcomponents associated with the motors 116, 118. For example, the speedsensors 138 may include one or more hall effect sensors (not shown). Insome embodiments, the one or more hall effect sensors are associatedwith each of the right motor 116 and the left motor 118.

In the illustrated embodiment, the control system 134, which may beconfigured to perform certain control functions, is operativelyconnected to the electric drive system 102 through the data linkinterface 136. The data link interface 136 may represent one or moreinterface devices that interconnect one or more data links with thecontrol system 134. It is contemplated that the data link interface 136may include other standard data links and may be configured in a mannerdifferent from the illustrated embodiment without departing from theteachings of this disclosure.

The control system 134 is operatively connected to an operator interface140 that may include a plurality of operator input devices such as, forexample, a steering device 142, an accelerator pedal or throttle 144, ashift lever 146, a retarder lever 148, and a display 150 forcommunicating information and commands between the operator and thecontrol system 134. The steering device 142 may be configured or adaptedto control the direction of travel of the machine 100 by controlling,for example, a steering angle of the travel mechanisms 104. In someembodiments, the steering device 142 may be actuated by electrical,mechanical, or hydraulic power.

In the illustrated embodiment, the steering device 142 is hydraulicallyactuated and may include known hydraulic and/or electrical componentsthat may cause one or more linkages to pivotally move to change asteering angle of the machine 100. The operator interface 140 mayinclude a steering angle sensor 152 associated with the steering device142 and adapted or configured to measure the steering angle of thetravel mechanisms 104, and thus, the steering angle of the machine 100.

In some embodiments, the operator interface 140 may include anaccelerator pedal position sensor 154 that is associated with theaccelerator pedal 144, which is used to determine a requested enginespeed that corresponds to a desired motor power. In some embodiments,the desired motor power may correspond with a depression of theaccelerator pedal 144. The accelerator pedal 144 may be configured tocontrol an acceleration and/or deceleration of the machine 100. Theaccelerator pedal position signal may be transmitted from theaccelerator pedal position sensor 154 to the other components of thecontrol system 134 to indicate an amount of torque requested by theoperator.

The control system 134 may control the electric drive system 102 toproduce a desired propulsion of the machine 100 in the forward or thereverse driving directions. The control system 134 may manage torquecommands for the motors 116, 118 by taking into account a number offactors, such as operator requests, current machine speed, engine poweravailability, machine speed limits, and environment factors, includingdrivetrain and component temperatures. In some embodiments, the controlsystem 134 may determine a desired torque to transmit to the motors 116,118 based on one or more of the accelerator pedal position signal, arequested gear command signal from the shift lever 146, a retarder leverposition signal, a payload status, and/or speed limits.

For example, the operator interface 140 may include a shift leverposition sensor 156 associated with the shift lever 146 to detect anoperator's intention to change from one position of the shift lever 146to another position of the shift lever 146. The requested gear commandsignal may represent such gear selections as park, reverse, neutral,drive, or low. The operator may engage the shift lever 146 to controlthe driving direction of the machine 100. For example, the shift lever146 may include at least a drive and a reverse position associatedrespectively with the forward and reverse driving directions of themachine 100.

The control system 134 may operatively interact with the operatorinterface 140 and other components to determine the ground speed of themachine 100. For example, the control system 134 may determine theground speed of a centerline of the machine 100 based at least in parton the rotational speed of at least one of the idling wheels 105 and thesteering angle of the machine 100. Nevertheless, it is contemplated thatany suitable method may be used to determine the travel or ground speedof the machine 100.

In the illustrated embodiment, the control system 134 includes one ormore data structure, such as, for example, one or more maps, which mayinclude two dimensional arrays or lookup tables, in memory. The maps maycontain data in the form of equations, tables, or graphs. The controlsystem 134 may contain a map that correlates a steering angle value to aslip or slide ratio. The control system 134 may be configured or adaptedto calculate a specific slip/slide ratio that corresponds to a specificsteering angle, and may further perform this calculation continuously asthe steering angle changes during operation.

The slip and slide ratios are non-dimensional values indicative ofrelative speeds between two wheels that are connected to the same axleor that are connected to the machine 100 at opposing sides. For example,the slip ratio may be a ratio of rotational speeds between right andleft rear wheels 106 (i.e., rotational speed of the right rear wheel 106divided by the rotational speed of left rear wheel 106), which should beabout equal to 1 when no slip/slide is present and the machine 100 istravelling in a straight line.

The control system 134 may use the slip/slide ratio map in an algorithmthat is adapted to adjust the torque commanded to each individual drivenwheel 106, 106. A flowchart for a method of controlling traction byadjusting the torque commanded to each individual driven wheel 106, 106is generally shown at 200 in FIG. 2. The control system 134 is arrangedfor simultaneous control of two driven wheels, each of which is drivenby a respective motor 116, 118. The wheels are designated as “right” or“left” to indicate that they are arranged on either side of the machine100 along a single axle. One can appreciate that the methods disclosedherein are equally applicable for machines having more or fewer than twodriven wheels.

As shown in FIG. 2, the control system 134 receives inputs from varioussystems of the machine 100, for example, an input from the steeringangle sensor 152. The control system 134 may also receive inputs from atleast one of the speed sensors 138. Based on these inputs, the controlsystem 134 is adapted to calculate and apply torques to the motors 116and 118 to control traction of the machine 100. The control system 134may use signals from the speed sensors 138 that are associated with eachof the driven wheels 106, 106 to determine the rotational speeds of eachdriven wheel 106 individually. A wheel speed of the left driven wheel106 is determined at 202 based on input from the at least one speedsensor 138.

The control system 134 may use a signal from the steering sensor 152 toaccount for machine turns in controlling the traction of the machine100. In the illustrated embodiment, the control system 134 is adapted orconfigured to determine a steering angle of the machine at 204 based ona signal from the steering angle sensor 152. The control system 134 mayuse signals from the speed sensors 138 that are associated with thenon-driven wheels 105, 105 to determine the ground speed of the machine100. The travel speed or ground speed of the machine is measured at 206,and the wheel speed of the right driven wheel is determined at 208 basedon input from the at least one speed sensor 138.

The control system 134 may determine a normalized or corrected speed orspeed ratio for each one of the driven wheels 106, 106. For example, aspeed ratio for the left driven wheel, V_(L,TS), may be calculated at210 by dividing the wheel speed for the left driven wheel 106, which wascalculated at 202, by the ground speed of the machine 100, which wascalculated at 206. In addition, the control system 134 may calculate aspeed ratio V_(R,TS) for the right driven wheel 106 at 212 by dividingthe wheel speed for the right driven wheel 106, which was calculated at208, by the ground speed of the machine, which was calculated at 206.

These normalizations or corrections of the drive wheels' speeds shouldbe equal to 1 when the machine speed or ground speed matches the speedof each wheel 106, that is, when there is no slipping or sliding, andshould change to a value above or below 1 when there is slipping orsliding. As can be appreciated, each speed ratio V_(R,TS) and V_(L,TS)will increase above 1 when the ground speed of the corresponding wheel106, 106 is greater than the speed of the machine 100, such as, forexample, when that wheel 106, 106 is slipping for lack of grip with theground, and will be less than 1 when the machine is travelling fasterthan the speed of the corresponding wheel 106, 106, such as, forexample, when the wheel 106, 106 is becoming stuck or when the wheel106, 106 is sliding during retarding.

The control system 134 also receives information indicative of the angleof the steering device 142 via a signal from the steering angle sensor152. The steering angle information is input to a table at 214 todetermine the expected slip or slide ratio, SR_(E), or the expected slipor slide that results when the machine 100 is turning and the wheels106, 106 arranged along a single “axle” line are following circularpaths that are at different distances from a center point of the turningradius of the machine 100. In other words, the expected slip ratioSR_(E) accounts for differences in rotational speed for the wheels 106,106 that are not mechanically linked to each other.

When the machine is turning, the steering angle determined at 204 isused to calculate an expected slip or slide ratio SR_(E) at 214. Thecalculation of the expected slip or slide ratio SR_(E) at 214 mayinclude a lookup table of slip/slide ratio versus steering angle or maybe any other type of calculation, such as, for example, a functionhaving the steering angle and slip/slide ratio values as variables.Nevertheless, the expected slip or slide ratio SR_(E) as well as thespeed ratios V_(L,TS) and V_(R,TS) are non-dimensional or normalizedparameters. Specifically, the expected slip or slide ratio SR_(E)represents the expected slip or slide or difference in wheel speed thatwill occur when the machine 100 is turning.

The expected slip or slide ratio SR_(E) is considered as the ratiobetween the speed of a wheel following an inner path of the turn and thespeed of the corresponding wheel following an outer path of the turn.For example, when the machine is turning left, the left driven wheel 106will follow an inner path that may be curved or circular about a turncenter (not shown), while the right driven right wheel 106 will followan outer path that is disposed at a greater radial distance from theturn center relative to the radial distance of the left driven wheel106.

The control system 134 uses the expected slip or slide ratio SR_(E) toperform a second normalization or correction of the speed ratiosV_(R,TS) and V_(L,TS) to account for steering. For example, when themachine 100 is turning, one or both speed ratios V_(R,TS) and V_(L,TS)may change from the base value of 1, even though there may be noslippage due to loss of fraction. This change may be the result of thedifferent trajectories followed by the driven wheels 106, 106 during theturn. In this situation, the expected slip or slide ratio SR_(E) can beused to account for the differences in wheel speed that are attributedto the turn, such that the respective speed ratio V_(R,TS) and V_(L,TS)for each driven wheel can be adjusted to the base value of 1 during theturn.

For example, the speed ratio of the wheel travelling on the inside trackduring a sharp turn may assume a speed ratio of one-half (½), indicatingthat the wheel is travelling at half the speed of the machine 100. Theexpected slip or slide ratio SR_(E) that corresponds to the specificturn angle may also be set to one-half (½), such that the ratio betweenthe speed ratio and the expected speed ratio is equal to 1. Hence, theresult of each of these normalizations is a corrected speed ratio, whichis calculated for each driven wheel 106.

In the illustrated embodiment, a left wheel corrected speed ratioV_(L,TS,SR) is calculated at 216 by dividing the speed ratio V_(L,TS)(210) for the left wheel 106 by the expected slip or slide ratio SR_(E)(214). Similarly, a right wheel corrected speed ratio V_(R,TS,SR) iscalculated at 218 by dividing the speed ratio V_(R,TS) (212) for theright wheel 106 by the expected slip or slide ratio SR_(E) (214). Boththe left wheel corrected speed ratio V_(L,TS,SR) and the right wheelcorrected speed ratio V_(R,TS,SR) represent non-dimensional values thatare indicative of slipping or sliding of the machine's driven wheels106, 106 during either straight line or turning motion of the machine100.

The corrected slip ratios V_(R,TS,SR) and V_(L,TS,SR) are not values ofactual slip or slide. Instead, the corrected slip ratios V_(R,TS,SR) andV_(L,TS,SR) are non-dimensional slip or slide parameters or ratios thatqualify and quantify a slip or slide condition for the driven wheels106, 106 disposed along the same drive axle of the machine 100. Thecorrected speed ratios V_(R,TS,SR) and V_(L,TS,SR) are inclusive oraccount for any straight-line motion slip or slide, which may be due touneven traction, as well as speed differentials in the driven wheels106, 106 that can result from turning.

Having determined the corrected speed ratios V_(R,TS,SR) andV_(L,TS,SR), the control system 134 compares each to a speed ratiothreshold value, T_(SR). The speed ratio threshold value T_(SR) may beconsidered as a threshold slip or slide condition that the machine 100may tolerate during operation in propulsion or retarding modes ofoperation. Each corrected speed ratio V_(L,TS,SR) and V_(R,TS,SR) iscompared to the threshold value T_(SR) individually such that the slipor slide of each driven wheel 106, 106 can be determined separately. Thethreshold value T_(SR) can be a constant, non-dimensional parameter, forexample, 10 percent (%), which represents the extent of slipping orsliding that can be present in the operation of the machine 100 withoutrequiring intervention by the control system 134 to the torquescommanded to each of the driven wheels 106, 106. The threshold T_(SR)may alternatively be a variable that depends on an operating parameterof the machine, for example, the ground speed of the machine 100.

In the illustrated embodiment, the threshold T_(SR) is determined at 220based on the ground speed of the machine 100 (206) using, for example, alookup table. The threshold T_(SR) is compared to each corrected speedratio V_(L,TS,SR) and V_(R,TS,SR) at, respectively, 222 and 224. Basedon the comparisons at 222 and 224, the control system 134 makes twoindependent determinations of whether one or both corrected speed ratiosV_(L,TS,SR) and V_(R,TS,SR) exceed the threshold T_(SR). When thecontrol system 134 determines that at least one corrected speed ratioV_(L,TS,SR) and/or V_(R,TS,SR) has exceeded the threshold T_(SR), thecontrol system 134 intervenes to adjust the torque being commanded tothe wheel that is slipping or sliding, by adjusting the torque beingcommanded at 226 and/or 228 to the corresponding motor 116 and/or 118.

The control system 134 may operate at a preset frequency or cycle time,for example, at 125 Hz. At each cycle, the control system 134 maycompare each of the corrected speed ratios V_(R,TS,SR) and V_(L,TS,SR)with the threshold value T_(SR) to determine whether a slip or slidecondition is present and whether the slip or slide condition exceeds theallowable slip or slide threshold for the ground speed of the machine100. When one or both of the corrected speed ratios V_(R,TS,SR) andV_(L,TS,SR) are determined to be higher than the calculated thresholdvalue T_(SR), the control system 134 may adjust the torque commanded tothe corresponding wheel, for example, by decreasing the torque beingcommanded to that wheel 106.

This adjustment to the speed of rotation of a corresponding wheel 106,106 purports to bring each corresponding corrected speed ratio to avalue that is within the threshold value T_(SR). In this embodiment, thecontrol system 134 may assume a more active role in reducing slip orslide of the driven wheels 106, 106 during operation. The control system134 continuously calculates a slip or slide ratio error or,alternatively, a difference between each corrected slip or slide ratio,V_(R,TS,SR) and V_(L,TS,SR) and the threshold value T_(SR). Stateddifferently, the continuously calculated corrected slip or slide ratiosV_(R,TS,SR) and V_(L,TS,SR) may be considered as “actual” slip or slideratios that are reflective of a slip or slide condition for each of thedriven wheels 106, 106.

These actual slip or slide ratios V_(R,TS,SR) and V_(L,TS,SR) shouldalways be within an acceptable range, which depends on the thresholdvalue T_(SR). Here, the control system 134 calculates a differencebetween each corrected speed ratio V_(L,TS,SR) and V_(R,TS,SR) and thethreshold T_(SR) to generate an error. The error may be used to drive aPI controller (not shown) but that is included within, respectively, 226and 228. The control system 134 may further include various othersub-routines or power circuits that command a torque to each motor at230 and 232.

In some embodiments, the control system 134 may include one or morecontrollers. In some embodiments, the one or more controllers mayinclude one or more control modules (e.g. ECMs, ECUs, etc.). The one ormore control modules may include processing units, memory, sensorinterfaces, and/or control signal interfaces (for receiving andtransmitting signals). The processing unit may represent one or morelogic and/or processing components used by the control system 134 toperform certain communications, control, and/or diagnostic functions.For example, the processing unit may be configured to execute routinginformation among devices within and/or external to the control system134.

Further, the processing unit may be configured to execute instructionsfrom a storage device, such as memory. The one or more control modulesmay include a plurality of processing units, such as one or more generalpurpose processing units and or special purpose units (for example,ASICS, FPGAs, etc.). In some embodiments, functionality of theprocessing unit may be embodied within an integrated microprocessor ormicrocontroller, including integrated CPU, memory, and one or moreperipherals or in multiple microprocessors or microcontrollers. Thememory may represent one or more known systems capable of storinginformation, including, but not limited to, a random access memory(RAM), a read-only memory (ROM), magnetic and optical storage devices,disks, programmable, erasable components such as erasable programmableread-only memory (EPROM, EEPROM, etc.), and nonvolatile memory such asflash memory.

INDUSTRIAL APPLICABILITY

The industrial applicability of the systems and methods for controllingtraction in an electric drive machine described herein will be readilyappreciated from the foregoing discussion. In accordance with certainembodiments, the disclosed control system may be applicable to anymachine that has wheels driven independently from each other, forexample, a machine having an electric or hydrostatic drive system thatuses a motor connected to each wheel. Each of the motors may be operatedindependently and without mechanical connections with other motors. Thedisclosed control system may be helpful in situations where one or bothof the driven wheels of the machine are slipping or sliding due to, forexample, poor traction when the machine is travelling in a straightline, in a forward or reverse driving direction, when the machine isturning, when the machine is operating in a retarding mode, such as, forexample, when braking, or any other conditions that cause differentialspeeds to occur in the driven wheels.

FIG. 3 illustrates an exemplary embodiment of the control system 134 andthe process (300) of adjusting an electric motor adapted to providebraking torque to at least one wheel until a speed ratio based in parton the speed of the at least one wheel is equal to an allowable slidethreshold during retarding. The control system 134 may determine thatmachine is in a retarding mode of operation (Step 302), such as, forexample, when an operator engages the retarder lever 148. The controlsystem 134 is adapted to determine a ground speed (Step 304). In someembodiments, the ground speed is an estimate of the travel speed of themachine 100 made based on speed measurements of traction motors, or onone or more independent ground speed measurements. In the illustratedembodiment, at least one of the speed sensors 138 is associated with thenon-driven wheels 105, and the ground speed measurements are based onthe measured speeds of the non-driven wheels 105 combined with thedriven wheels 106, as discussed in detail above. For convenience, allspeeds are expressed in relation to traction motor RPM, although it iscontemplated that the speeds can be expressed in wheel RPM or machinespeed in km/hr.

The control system 134 further determines an amount of slide that willbe allowable (Step 306) before an attempt to correct or adjust isinitiated. In the illustrated embodiment, the allowable wheel slidethreshold is established for use during retarding and is setapproximately below the ground speed of the machine 100. For example, insome embodiments, the allowable slide threshold may be established as apercentage of ground speed. If, for example, a ten percent (10%) slidemay be allowed, and the measure ground speed corresponds to 1000 RPM onthe traction motors, than any motor speed down to 900 RPM would beallowed without correction. Thus, any motor speed less than 900 RPM willbe below the threshold.

The control system 134 determines the motor speed of at least one of thedriven wheels 106 (Step 308). In some embodiments, the control system134 may determine the rotational speeds of each of the driven wheels106, 106 to compare to the slide threshold. The actual motor speed (fromStep 308) is compared (Step 310) to the allowable slide threshold (fromStep 306). If the motor speed is less than the slide threshold (Step310; Yes), the control system 134 is adapted to adjust the torqueprovided to the motor associated with the wheel until the motor speed iswithin to the slide threshold.

In the illustrated embodiment, control system 134 controls each motorindependently so that each wheel speed is separately controlled and eachwheel speed is essentially maintained substantially equal to the other.In some embodiments, if the motor speed is less than the slidethreshold, the control system 134 is adapted to hold mechanical braketorque constant while modulating the electric motor torque until thewheel speed is within the slide threshold.

It will be appreciated that the foregoing description provides examplesof the disclosed system and technique. However, it is contemplated thatother implementations of the disclosure may differ in detail from theforegoing examples. All references to the disclosure or examples thereofare intended to reference the particular example being discussed at thatpoint and are not intended to imply any limitation as to the scope ofthe disclosure more generally. All language of distinction anddisparagement with respect to certain features is intended to indicate alack of preference for those features, but not to exclude such from thescope of the disclosure entirely unless otherwise indicated.

Recitation of ranges of values herein are merely intended to serve as ashorthand method of referring individually to each separate valuefalling within the range, unless otherwise indicated herein, and eachseparate value is incorporated into the specification as if it wereindividually recited herein. All methods described herein can beperformed in any suitable order unless otherwise indicated herein orotherwise clearly contradicted by context.

Accordingly, this disclosure includes all modifications and equivalentsof the subject matter recited in the claims appended hereto as permittedby applicable law. Moreover, any combination of the above-describedelements in all possible variations thereof is encompassed by thedisclosure unless otherwise indicated herein or otherwise clearlycontradicted by context.

1. A traction control system for a machine having an electric driveconfiguration, comprising: an electric motor associated with at leastone wheel and adapted to provide braking torque to the wheel; and acontroller configured to: determine a rotational speed of the at leastone wheel; compare the rotational speed to an allowable slide threshold;and adjust the braking torque to the at least one wheel during retardingif the speed is less than the allowable slide threshold.
 2. The tractioncontrol system of claim 1, wherein the allowable slide threshold isbased on a ground speed of the machine.
 3. The traction control systemof claim 2, wherein the allowable slide threshold is a percentage of theground speed of the machine.
 4. The traction control system of claim 3,wherein the allowable slide threshold is ninety percent of the measuredground speed of the machine.
 5. The traction control system of claim 1,further including at least one mechanical brake associated with the atleast one wheel, wherein the mechanical brake is adapted to providebraking torque to the wheel.
 6. The traction control system of claim 5,wherein the controller is further configured to: adjust the electricmotor to apply a first portion of the braking torque to the at least onewheel during retarding; and adjust the mechanical brake to provide asecond portion of the braking torque to the at least one wheel duringretarding until the speed is greater than the allowable slide threshold.7. The traction control system of claim 6, wherein the second portion ofthe braking torque is held constant while the first portion of thebraking torque is modulated until the speed is greater than theallowable slide threshold.
 8. A method for controlling traction of amachine having an electric drive configuration, comprising: determininga retarding mode of operation for the machine; determining a rotationalspeed of at least one wheel of the machine during retarding; comparingthe rotational speed to an allowable slide threshold; and adjusting abraking torque provided by an electric motor to the at least one wheelif the speed is less than the allowable slide threshold.
 9. The methodof claim 8, further comprising: determining a ground speed of themachine during retarding; and comparing the rotational speed to anallowable slide threshold based on the ground speed.
 10. The method ofclaim 9, wherein comparing the rotational speed to an allowable slidethreshold includes the allowable slide threshold based on a percentageof the ground speed.
 11. The method of claim 10, wherein comparing therotational speed to an allowable slide threshold includes the allowableslide threshold based on ninety percent of the ground speed.
 12. Themethod of claim 8, further comprising: adjusting the braking torqueincludes adjusting braking torque provided by a mechanical brake. 13.The method of claim 12, wherein adjusting the braking torque includesadjusting a first portion of the braking torque provided by the electricmotor and adjusting a second portion of the braking torque provided bythe mechanical brake until the speed is greater than the allowablethreshold.
 14. The method of claim 13, wherein adjusting the secondportion includes holding the braking torque provided by the mechanicalbrake constant and adjusting the second portion includes modulating thebraking torque provided by the electric motor until the speed is greaterthan the allowable threshold.
 15. An electric drive machine, comprising:a first wheel; a first motor operating to rotate the first wheel about afirst axis relative to the machine; a first sensor disposed to measure aspeed of the first wheel relative to the machine; a speed sensordisposed to measure the speed of the machine relative to the ground; acontroller operatively connected to the first motor, the first sensor,and the speed sensor; the controller configured to: determine arotational speed of the first wheel based on information from the firstsensor; determine a ground speed of the machine based on informationfrom the speed sensor; determine a slide threshold value based on theground speed; compare the rotational speed with the threshold value; andwhen the rotational speed is less than the threshold value, adjust atorque command to the first motor such that the rotational speed exceedsthe threshold value.
 16. The machine of claim 15, further comprising: asecond wheel; a second motor operating to rotate the second wheel abouta second axis relative to the machine, wherein the second axis isparallel to the first axis; a second sensor disposed to measure a speedof the second wheel relative to the machine; wherein the controller isfurther configured to: determine a rotational speed of the second wheelbased on information from the second sensor; determine a slide thresholdvalue based on information from the speed sensor; compare the rotationalspeed with the threshold value; and when the rotational speed is lessthan the threshold value, adjust a torque command to the second motorsuch that the second rotational speed exceeds the threshold value. 17.The machine of claim 16, further comprising: a first mechanical brakeoperatively coupled to the first wheel, the first mechanical brakeconfigured to providing braking torque to the wheel.
 18. The machine ofclaim 17, further comprising: a second mechanical brake operativelycoupled to the second wheel, the second mechanical brake configured toproviding braking torque to the wheel.
 19. The machine of claim 17,wherein the controller is further configured to: provide constantbraking torque from the first mechanical brake; and adjust the torquecommand to the first motor to modulate the braking torque until thefirst rotational speed exceeds the threshold value.
 20. The machine ofclaim 19, wherein the controller is further configured to: provideconstant braking torque from the second mechanical brake; and adjust thetorque command to the second motor to modulate the braking torque untilthe second rotational speed exceeds the threshold value.